1. Field of the Invention
The present invention relates to a mobile radiophone apparatus, and more particularly to a digital mobile radiophone apparatus including a highly efficient voice encoder in which an input analogue voice signal is quantized through an A/D conversion and features of the quantized voice signal are extracted to convert the signal to codes of a smaller data quantity.
2. Description of the Related Art
In a highly efficient voice-encoding technique for voice signals in a digital mobile radiophone apparatus, after an input signal such as a voice is quantized through an A/D conversion, the quantized signal is divided into fixed units along the time axis and a group of plural quantization samples is encoded together. For encoding, there are methods such as linear estimation analysis in which calculation is performed by using a parameter representing a feature in time of the voice signal as a coefficient for filtering (refer to "DIGITAL MOBILE RADIOPHONE SYSTEM STANDARD RCR-STD-27B", p.515, issued by, a foundation, Denpa System Kaihatu Center, for instance).
By the way, the analogue voice signal input to the mobile radiophone apparatus is quantized through the A/D conversion and then input to the highly efficient voice encoder. But the amplitude data quantized through the A/D conversion are input to the highly efficient voice encoder as they are, in general.
FIG. 1 is a block diagram showing a conventional mobile radiophone apparatus 2 in a digital communication system, in respect of conversion an analogue voice signal input as a voltage amplitude to highly efficient voice codes and conversion of highly efficient voice codes to an analogue voice signal.
As shown in FIG. 1, an analogue voice input to a microphone 25 of the digital mobile radiophone apparatus 2 is input to an A/D converter 21 as a voltage amplitude, converted to quantized data representing the voltage amplitude in the A/D converter 21, and then input to a highly efficient voice encoder 22. The highly efficient voice encoder 22 extracts their features in time from the quantized data representing the voltage amplitude, performs highly efficient voice-encoding, and then sends out the encoded codes to a transmitter 28 as transmission voice codes. The transmission voice codes are modulated in the transmitter 28 and then transmitted through an antenna 29 to a base station.
On the other hand, modulated highly efficient voice codes received from the base station through the antenna 29 of the digital mobile radiophone apparatus 2 are demodulated in a receiver 27 and then input to a highly efficient voice decoder 23. The highly efficient voice decoder 23 decodes the codes to quantized data representing a voltage amplitude and then sends out them to a D/A converter 24. The data are converted to the voltage amplitude in the D/A converter 24 and then output through a speaker 26 as an analogue voice.
As described above, in the conventional highly efficient voice-encoding method in the mobile radiophone apparatus, because the quantized data A/D-converted are input to the highly efficient voice encoder as they are, the amplitude of the voice signal is small. In the case of a little variation of the quantization samples, there is such problem that faithfulness to the original input voice signal decreases upon decoding the voice-encoded data. This is because calculation error upon performing highly efficient voice-encoding is large owing to a small quantity of information included by the input voice signal or the like in the case of a little variation of the samples.